Since the advent of a resist for KrF excimer laser (248 nm), an image forming method called chemical amplification is used as an image forming method for a resist so as to compensate for sensitivity reduction caused by light absorption. For example, the image forming method by positive chemical amplification is an image forming method of decomposing an acid generator in the exposed area upon exposure with excimer laser, electron beam, extreme-ultraviolet light or the like to produce an acid, converting an alkali-insoluble group into an alkali-soluble group by using the generated acid as a reaction catalyst in the baking after exposure (PEB: Post Exposure Bake), and removing the exposed area with an alkali developer.
As for the alkali developer used in the method above, various alkali developers have been proposed, but an aqueous alkali developer of 2.38 mass % TMAH (an aqueous tetramethylammonium hydroxide solution) is being used for general purposes.
Also, due to miniaturization of a semiconductor device, the trend is moving into a shorter wavelength of the exposure light source and a higher numerical aperture (high NA) of the projection lens, and an exposure machine using an ArF excimer laser with a wavelength of 193 nm as a light source has been developed at present. Furthermore, a so-called immersion method of filling a high refractive-index liquid (hereinafter sometimes referred to as an “immersion liquid”) between the projection lens and the sample is being aggressively studied. The immersion method can be combined with the super-resolution technology under study at present, such as phase-shift method and modified illumination method.
As for the technique to more enhance the resolution, double exposure technology or double patterning technology has been proposed.
In conventional pattern formation of an electronic device such as semiconductor device, a mask or reticle pattern enlarged to 4 to 5 times the size of a pattern intended to form is size-reduced and transferred on an exposure target such as wafer by using a reduction projection exposure apparatus.
However, the dimensional miniaturization brings about a problem that in the conventional exposure system, lights irradiated on adjacent patterns interfere with each other to decrease the optical contrast. Therefore, in such technology, an approach of dividing the exposure mask design into two or more parts and synthesizing an image by independently exposing these masks is being taken. In this double exposure system, it is necessary to divide the exposure mask design and again synthesize an image of the design on an exposure target (wafer), and therefore, division of the mask design must be devised so that the pattern on the reticle can be faithfully reproduced on the exposure target.
Studies of applying the effect of such a double exposure system to the transfer of a fine image pattern of a semiconductor device are introduced, for example, in JP-A-2006-156422 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”).
In the case of simply applying the conventional resist composition to the double exposure system, the pattern formation needs to be performed near the resolution limit of the resist, and this incurs a problem that sufficient exposure margin or depth of focus cannot be obtained.
JP-A-2000-199953 describes a double development technology as the double patterning technology for enhancing the resolution. In this example, an image forming method by general chemical amplification is utilized, and by making use of a property that the polarity of a resin in a resist composition when exposed becomes high in a region irradiated with a high light intensity and maintained low in a region irradiated with a low light intensity, the high exposure region of a specific resist film is dissolved with a high-polarity developer (specifically, a conventional aqueous alkali solution) (positive development) and the low exposure region is dissolved with a low-polarity developer (specifically, an organic solvent) (negative development). More specifically, the region not lower than an exposure dose E2 of irradiation light 1 shown in FIG. 1 is dissolved using an aqueous alkali solution, and the region not higher than an exposure dose E1 is dissolved using a specific organic solvent, whereby, as shown in FIG. 1, the region with a medium exposure dose (E2-E1) is allowed to remain without being developed and an L/S pattern 3 having a pitch half the pitch of the exposure mask 2 is formed on a wafer 4.
However, it is very difficult to select an optimal combination of a resist composition and an organic solvent developer, and there is a problem that developability when using an organic solvent developer becomes bad.
Furthermore, at the formation of a fine pattern by double development, merely showing good resolution on use of an organic solvent or an aqueous alkali solution alone is not enough, and it is demanded to show good pattern resolution for both developers.
Considering these problems, in the double development technology, JP-A-2008-292975 proposes a pattern forming method using a specific resist composition. According to this technique, a high-precision fine pattern is supposed to be stably obtained.
However, more improvements are demanded in the sensitivity, exposure latitude (EL) and other various performances.